Satellite radio operators are providing digital radio broadcast services covering the entire continental United States. These services offer approximately 100 channels, of which nearly 50 channels in a typical configuration provides music with the remaining stations offering news, sports, talk and data channels. Briefly, the service provided by XM Satellite Radio includes a satellite X-band uplink to two satellites which provide frequency translation to the S-band for re-transmission to radio receivers on earth within a coverage area. Radio frequency carriers from one of the satellites are also received by terrestrial repeaters. The content received at the repeaters is retransmitted at a different S-band carrier to the same radios that are within their respective coverage areas. These terrestrial repeaters facilitate reliable reception in geographic areas where LOS reception from the satellites is obscured by tall buildings, hills, tunnels and other obstructions. The signals transmitted by the satellites and the repeaters are received by satellite digital audio radio system (SDARS) receivers which can be located in automobiles, in handheld or in stationary units for home or office use. The SDARS receivers are designed to receive one or both of the satellite signals and the signals from the terrestrial repeaters and combine or select one of the signals as the receiver output.
Hierarchical modulation and demodulation is well known in fixed environments such as satellite and terrestrial systems. For example, the Digital Video Broadcasting specification (in Europe) for terrestrial signaling (DVB-T) is a flexible system allowing terrestrial broadcasters to choose from a variety of options to suit their various service environments and generally enables such broadcasters to trade-off bit-rate versus signal robustness.
In hierarchical modulation as described in the DVB-T specification, two separate datastreams are modulated onto a single DVB-T stream. One stream, called the “High Priority” (HP) stream is embedded within a “Low Priority” (LP) stream. Receivers with “good” reception conditions can receive both streams, while those with poorer reception conditions may only receive the “High Priority” stream. Broadcasters can target two different types of DVB-T receivers with two completely different services. Typically, the LP stream is of higher bitrate, but lower robustness than the HP one. For example, a broadcaster could choose to deliver HDTV in the LP stream.
DVB-T is a multi-carrier system using about 2000 or about 8000 carriers, each of which carries QPSK, 16QAM or 64QAM. QAM (Quadrature Amplitude Modulation) is one of the means used to increase the amount of information per modulation symbol. Taking the example of 64QAM, a hierarchical system can map data onto 64QAM in such a way that there is effectively a QPSK stream buried within the 64QAM stream. Further, the spacing between constellation states can be adjusted to protect the QPSK (HP) stream, at the expense of the 64QAM (LP) stream.
In layman's terms, good quality reception allows receivers to resolve the entire 64QAM constellation. In areas with poorer quality reception, or in the case of mobile or portable reception, receivers may only be able to resolve the lighter colored portions of the constellation, which correspond to QPSK. Considering bits and bytes, in a 64QAM constellation you can code 6 bits per 64QAM symbol. In hierarchical modulation, the 2 most significant bits (MSB) would be used for the robust mobile service, while the remaining 6 bits would contain, for example, a HDTV service. The first two MSBs correspond to a QPSK service embedded in the 64QAM one. To date, no existing system is known to have combined hierarchical data streams from both a satellite data stream and a terrestrial data stream and certainly not for digital audio radio systems.